In magnetic recording of television signals, present technological limitations on the recording process limit the frequency range of the recorded signal to 10 octaves. However, the video signal spectrum encompasses 17-18 octaves. Consequently, compression of the video signal bandwidth is necessary in order to record all the information contained within the signal. Frequency modulation is one method of compressing the bandwidth of a signal and pulse frequency modulation has been put to significant use in the professional video recording industry.
High quality is required in all television signal recording applications. Moreover, the broadcasting industry has developed rigid standards with respect to signal quality. Accordingly, the fm modulators utilized in video tape recorders must provide a minimum of signal distortion and a maximum of fidelity. Such modulators are complex in design and expensive to construct.
One such modulator iss utilized in the model AVR-1 video tape recorder manufactured by the Ampex Corporation. That modulator is a pulse modulator which utilizes a form of relaxation oscillator that incorporates a tunnel diode. The tunnel diode operates as a bistable device that is switched between its stable states to control capacitor charge and discharge cycles. The capacitor is charged and discharged by way of two separate current sources. A differential amplifier is also employed to control the exact points at which the oscillator changes states. The modulator is constructed of discrete elements utilizing specially ordered transistors and other high quality circuit elements. The tunnel diode itself is an expensive item. The circuit is complex and relatively expensive to construct.
The AVR-1 is a high quality pulse frequency modulator. However, even in the highest quality modulators a finite switching time is required for the oscillator and other circuit elements to change states. This switching time is a constant independent of operating frequency. At the high frequencies utilized in video tape recording, switching time constitutes a substantial part of the pulse width. Consequently, a nonlinear relationship between changes in the video signal and changes in modulated frequency can result. The signal will be distorted unless some compensation is provided to maintain a linear functional relationship.